In order to achieve efficient thermal conversion of solar energy in a solar collector, the collector element or elements should have a surface which has selective absorptance and reflectance. Thus, the surface should be capable of absorbing electromagnetic radiation of wavelength from about 0.3 .mu.m. to about 2.0 .mu.m. and of reflecting radiation which has a wavelength larger than about 2.0 .mu.m. This requirement is met by applying an absorbing film (usually a refractory) of thickness between 0.02 and 0.20 .mu.m. to a reflective surface coating of aluminium, silver, copper or other metal which has a thickness which is substantially non-transmissive of infra-red radiation.
The absorbing film preferably is graded in a manner such that its refractive index varies with thickness of the film. Thus, for example, in the case of a metal-carbide absorbing film, the volume ratio of the metal and carbon atoms is varied with depth of the film such that the metal atoms are more heavily concentrated at the interface of the absorbing film and the reflective surface or coating than at the outer surface of the film. Such films (including metal-carbides, metal-oxides, metal-nitrides, metal-silicides, etc.) which are deposited in a manner to provide for variation of refractive index with depth are generally and herein referred to as graded surface coatings.